This experiment demonstrates the concepts outlined in the Manitoba Middle Years Specific Learning Outcome of: 5-4-03 (GLO: D3)
That is, in this experiment, the students will describe the properties of air, specifically that air:
has mass/weight and volume
expands to fill a space
expands and rises when heated
contracts and sinks when cooled
moves from areas of high pressure to areas of low pressure
Students will be using a hot plate, so care should be taken to make sure that the students are careful while using it.
The students should also use tongs to handle the pop can after it has been heated, so as not to burn themselves.
Once the water is boiling, use the tongs to pick up the can and quickly flip it upside down into the basin of cold water so that the hole in the top of the can is completely submerged in the basin of water
The can will implode, crushing inward rather dramatically
This should create a sense of disequilibrium in the students, since most of them will not be expecting the can to spontaneously crush itself
Ask students “Why did that happen?”
Engage in some discussion with the students
After some discussion, explain the theory about what happened with the students to bring them back to a state of equilibrium
In the beginning, the pressure inside the can is equal to the pressure outside the can (the atmospheric pressure), as air can pass freely in and out of the can through the hole in the top. Once you pour water into the can, and begin to heat it, the pressure is still normalized through the open hole in the top, which allows air in and out of the can to maintain a constant pressure equal to the atmospheric pressure.
As the water boils inside the can, the water vapour created displaces the air that is inside the can. Once the water vapour cools by removing the can from the heat source and placing it in the cold water, it condenses and fewer water molecules collide with the sides of the can. This causes a decrease in pressure inside the can.
Since the pressure inside the can is now less than the atmospheric pressure outside, the air molecules in the atmosphere attempt to normalize that pressure by entering the can. Since the opening of the can is sealed by being dipped in the water, the air molecules push on the sides of the can until the volume of the can is reduced to a point where the pressure inside the can is equal to the pressure outside.
You used more/less water? - If there is no water in the can, so that you are only heating the air, the can will still be crushed, but not to the same degree that it would when you have added water. The reason for this is that the volume difference between heated and cooled steam is much greater than the volume difference between heated and cooled air. Therefore, when the can with the water is cooled, there is more of a pressure difference than if there is just air, and the can is crushed to a greater degree. The more water you have inside the can, the less the can will crush as well since you are decreasing the volume of air to be displaced inside the can.
What if you used a larger can? -If you used a larger can, it would still have the same effect because the steam would expand to occupy the entire volume of the can and displace the air that is present. For this example you could use the larger metal can to demonstrate what would happen. In this case, you would pour approximately 100 mL of water in the can, and heat the can until the water boils. Once the water boils, take the can off the hot plate, and put a cap over the spout of the can. Then place the can in the basin of cold water. The much larger can will also implode. You could repeat this same experiment using an oil drum.
Once upon a time there was a man named Ben. He was a simple guy who enjoyed the simple things in life. However, he was not particularly ambitious going through school, and often daydreamed in class instead of paying attention to his work. When he graduated, he couldn't decide what he wanted to do with his life, and so he wound up floating a lot through different careers.
Eventually he found what he thought was his true calling; cleaning out the insides of tank cars for trains. The pay wasn`t great, and typically the job was pretty dirty and smelly, but he enjoyed it. He didn`t have to interact a lot with other people, which was good, because he didn`t like interacting with people a lot. The job also didn`t require a lot of responsibility, which suited him just fine. For the most part, the job only involved was steam cleaning the inside of the tank car.
The only task he was responsible for was making sure that he left one of the lids off the tanker car when he finished cleaning it out. Ben never understood why it was that was, and thought it was a pretty silly rule, but the boss seemed to think it was pretty important, so he did it. However, Ben never considered it to be as important as his boss did.
Well, as you may have guessed, one day Ben decided to close the lids on the tank car because it was raining and he was afraid that the tanker would fill up with water. He closed all the outlet valves, and sealed the tank car completely. Ben then dropped off his equipment and went home for the evening.
The next day Ben arrived at work. He couldn`t see anyone around. When he walked out into the train yard, he noticed a bunch of people all gathered in a group at the far end of the yard. "That`s strange," Ben thought, "That`s right about where left the tank car last night." As Ben started to approach the mob, he noticed his boss standing there. The look on his face conveyed the fact that he was not impressed. When he saw Ben approach, he ran up to him quickly.
"What did you do?" his boss demanded, "Did you seal the tank car after you cleaned it yesterday?"
"I did," said Ben, "but it was raining and I didn't want it to fill up with water."
"Well, that won't be a problem anymore," said his boss.
Ben was puzzled, "What do you mean?"
"Have a look for yourself," said his boss.
Ben could hardly believe his eyes when he saw what had happened.
Perform the demonstration as per the description above.
At that moment, Encyclopedia Brown happened to be walking by and offered to explain what happened to the tank car. When Ben cleaned out the car using the steam cleaner, the steam displaced the air that was in the car. When the steam that was trapped inside the tank car cooled, it condensed, which resulted in the pressure inside the tank car being far less than the pressure outside the car. Since the pressure inside was lower, the air molecules outside the car attempted to normalize the pressure by entering the car. Since the car was completely sealed, there was no easy way for the air molecules to enter the car, and therefore they tried to enter the car through the walls of the tank car. Since they could not do that, the only option remaining was to reduce the interior volume of the car until the pressure inside the car was equal to the atmospheric pressure, which was accomplished by crushing the car.
Draw a picture of the device used to demonstrate the discrepant event below
Explain how it works.
Ben wished he had paid more attention in science class. "I bet that wouldn't have happened had I had just made sure that I left more water in the tank before I closed it up," Ben grumbled to himself as his boss yelled at him.
Carry out a fair test to determine if:
1) A change in the amount of water used to displace the air inside the can.
Step 1: Plan out your investigation
What will I change?What will I keep the same?What and how will I measure?
Why did the pop can implode when it was placed in the cold water?
How could knowledge of this lab help you to explain why the sides of a juice box collapse in on it as you suck on the straw?
You are an auto mechanic working in a garage in the middle of winter. A customer comes back complaining that you did not inspect his car closely enough and believes that the tires are leaking air. Upon questioning the customer, you find out that the customer had to leave the car with the garage for several days, where it was left in a heated bay. The tires appeared fine to the customer when he left, but shortly afterward, the tires appeared to be flatter. You look the tires over and there does not appear to be any holes where air could leak out. Having completed this lab, what could you tell the customer to put his mind at ease?
Can you think of any other examples of pressure changes in everyday life?
References: Schwagle, Jim (2005). Can Crushing. Retrieved October 11, 2008, from Discrepant Event - Teacher's Guide Web site: http://www.csun.edu/scied/4-discrpeant-event/the_can_crush/index.htm Bradburn, Gregory (2007, January). Can Crushing Experiment. Retrieved October 11, 2008, from Ask A Scientist Web site: http://www.newton.dep.anl.gov/askasci/chem99/chem99406.htm